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.\" This package is an SSL implementation written
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.Dd $Mdocdate: November 6 2016 $
.Dt LH_NEW 3
.Os
.Sh NAME
.Nm lh_new ,
.Nm lh_free ,
.Nm lh_insert ,
.Nm lh_delete ,
.Nm lh_retrieve ,
.Nm lh_doall ,
.Nm lh_doall_arg ,
.Nm lh_error
.Nd dynamic hash table
.Sh SYNOPSIS
.In openssl/lhash.h
.Fn DECLARE_LHASH_OF <type>
.Ft LHASH *
.Fn lh_<type>_new void
.Ft void
.Fo lh_<type>_free
.Fa "LHASH_OF(<type>) *table"
.Fc
.Ft <type> *
.Fo lh_<type>_insert
.Fa "LHASH_OF(<type>) *table"
.Fa "<type> *data"
.Fc
.Ft <type> *
.Fo lh_<type>_delete
.Fa "LHASH_OF(<type>) *table"
.Fa "<type> *data"
.Fc
.Ft <type> *
.Fo lh_<type>_retrieve
.Fa "LHASH_OF<type>) *table"
.Fa "<type> *data"
.Fc
.Ft void
.Fo lh_<type>_doall
.Fa "LHASH_OF(<type>) *table"
.Fa "LHASH_DOALL_FN_TYPE func"
.Fc
.Ft void
.Fo lh_<type>_doall_arg
.Fa "LHASH_OF(<type>) *table"
.Fa "LHASH_DOALL_ARG_FN_TYPE func"
.Fa "<type2>"
.Fa "<type2> *arg"
.Fc
.Ft int
.Fo lh_<type>_error
.Fa "LHASH_OF(<type>) *table"
.Fc
.Ft typedef int
.Fo (*LHASH_COMP_FN_TYPE)
.Fa "const void *"
.Fa "const void *"
.Fc
.Ft typedef unsigned long
.Fo (*LHASH_HASH_FN_TYPE)
.Fa "const void *"
.Fc
.Ft typedef void
.Fo (*LHASH_DOALL_FN_TYPE)
.Fa "const void *"
.Fc
.Ft typedef void
.Fo (*LHASH_DOALL_ARG_FN_TYPE)
.Fa "const void *"
.Fa "const void *"
.Fc
.Sh DESCRIPTION
This library implements type-checked dynamic hash tables.
The hash table entries can be arbitrary structures.
Usually they consist of key and value fields.
.Pp
.Fn lh_<type>_new
creates a new
.Vt LHASH_OF(<type>)
structure to store arbitrary data entries, and provides the hash and
compare callbacks to be used in organising the table's entries.
The hash callback takes a pointer to a table entry as its argument
and returns an unsigned long hash value for its key field.
The hash value is normally truncated to a power of 2, so make sure that
your hash function returns well mixed low order bits.
The compare callback takes two arguments (pointers to two hash table
entries), and returns 0 if their keys are equal, non-zero otherwise.
If your hash table will contain items of some particular type and the
hash and compare callbacks hash and compare these types, then the
.Fn DECLARE_LHASH_HASH_FN
and
.Fn IMPLEMENT_LHASH_COMP_FN
macros can be used to create callback wrappers of the prototypes
required by
.Fn lh_<type>_new .
These provide per-variable casts before calling the type-specific
callbacks written by the application author.
These macros, as well as those used for the doall callbacks, are
defined as;
.Bd -literal -offset 2n
#define	DECLARE_LHASH_HASH_FN(name, o_type) \e
	unsigned long name##_LHASH_HASH(const void *);
#define	IMPLEMENT_LHASH_HASH_FN(name, o_type) \e
	unsigned long name##_LHASH_HASH(const void *arg) { \e
		const o_type *a = arg; \e
		return name##_hash(a); }
#define	LHASH_HASH_FN(name) name##_LHASH_HASH

#define	DECLARE_LHASH_COMP_FN(name, o_type) \e
	int name##_LHASH_COMP(const void *, const void *);
#define	IMPLEMENT_LHASH_COMP_FN(name, o_type) \e
	int name##_LHASH_COMP(const void *arg1, const void *arg2) { \e
		const o_type *a = arg1;		    \e
		const o_type *b = arg2; \e
		return name##_cmp(a,b); }
#define	LHASH_COMP_FN(name) name##_LHASH_COMP

#define	DECLARE_LHASH_DOALL_FN(name, o_type) \e
	void name##_LHASH_DOALL(void *);
#define	IMPLEMENT_LHASH_DOALL_FN(name, o_type) \e
	void name##_LHASH_DOALL(void *arg) { \e
		o_type *a = arg; \e
		name##_doall(a); }
#define	LHASH_DOALL_FN(name) name##_LHASH_DOALL

#define	DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \e
	void name##_LHASH_DOALL_ARG(void *, void *);
#define	IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \e
	void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \e
		o_type *a = arg1; \e
		a_type *b = arg2; \e
		name##_doall_arg(a, b); }
#define	LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG
.Ed
.Pp
An example of a hash table storing (pointers to) structures of type
\&'STUFF' could be defined as follows;
.Bd -literal -offset 2n
/* Calculate the hash value of 'tohash' (implemented elsewhere) */
unsigned long STUFF_hash(const STUFF *tohash);
/* Order 'arg1' and 'arg2' (implemented elsewhere) */
int stuff_cmp(const STUFF *arg1, const STUFF *arg2);
/* Create type-safe wrapper functions for use in the LHASH internals */
static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF);
static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
/* ... */
int main(int argc, char *argv[]) {
        /* Create the new hash table using the hash/compare wrappers */
        LHASH_OF(STUFF) *hashtable =
	    lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
        	LHASH_COMP_FN(STUFF_cmp));
	/* ... */
}
.Ed
.Pp
.Fn lh_<type>_free
frees the
.Vt LHASH_OF(<type>)
structure
.Fa table .
Allocated hash table entries will not be freed; consider using
.Fn lh_<type>_doall
to deallocate any remaining entries in the hash table (see below).
.Pp
.Fn lh_<type>_insert
inserts the structure pointed to by
.Fa data
into
.Fa table .
If there already is an entry with the same key, the old value is
replaced.
Note that
.Fn lh_<type>_insert
stores pointers, the data are not copied.
.Pp
.Fn lh_<type>_delete
deletes an entry from
.Fa table .
.Pp
.Fn lh_<type>_retrieve
looks up an entry in
.Fa table .
Normally,
.Fa data
is a structure with the key field(s) set; the function will return a
pointer to a fully populated structure.
.Pp
.Fn lh_<type>_doall
will, for every entry in the hash table, call
.Fa func
with the data item as its parameter.
For
.Fn lh_<type>_doall
and
.Fn lh_<type>_doall_arg ,
function pointer casting should be avoided in the callbacks (see
.Sx NOTES )
\(em instead use the declare/implement macros to create type-checked
wrappers that cast variables prior to calling your type-specific
callbacks.
An example of this is illustrated here where the callback is used to
cleanup resources for items in the hash table prior to the hashtable
itself being deallocated:
.Bd -literal -offset 2n
/* Clean up resources belonging to 'a' (this is implemented elsewhere) */
void STUFF_cleanup_doall(STUFF *a);
/* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF)
	/* ... then later in the code ... */
/* So to run "STUFF_cleanup" against all items in a hash table ... */
lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
/* Then the hash table itself can be deallocated */
lh_STUFF_free(hashtable);
.Ed
.Pp
When doing this, be careful if you delete entries from the hash table in
your callbacks: the table may decrease in size, moving the item that you
are currently on down lower in the hash table \(em this could cause some
entries to be skipped during the iteration.
The second best solution to this problem is to set hash->down_load=0
before you start (which will stop the hash table ever decreasing in
size).
The best solution is probably to avoid deleting items from the hash
table inside a doall callback!
.Pp
.Fn lh_<type>_doall_arg
is the same as
.Fn lh_<type>_doall
except that
.Fa func
will be called with
.Fa arg
as the second argument and
.Fa func
should be of type
.Vt LHASH_DOALL_ARG_FN_TYPE
(a callback prototype that is passed both the table entry and an extra
argument).
As with
.Fn lh_<type>_doall ,
you can instead choose to declare your callback with a prototype
matching the types you are dealing with and use the declare/implement
macros to create compatible wrappers that cast variables before calling
your type-specific callbacks.
An example of this is demonstrated here (printing all hash table entries
to a BIO that is provided by the caller):
.Bd -literal -offset 2n
/* Print item 'a' to 'output_bio' (this is implemented elsewhere) */
void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio);
/* Implement a prototype-compatible wrapper for "STUFF_print" */
static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF, BIO)
	/* ... then later in the code ... */
/* Print out the entire hashtable to a particular BIO */
lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO,
	logging_bio);
.Ed
.Pp
.Fn lh_<type>_error
can be used to determine if an error occurred in the last operation.
.Fn lh_<type>_error
is a macro.
.Sh RETURN VALUES
.Fn lh_<type>_new
returns
.Dv NULL
on error, otherwise a pointer to the new
.Vt LHASH
structure.
.Pp
When a hash table entry is replaced,
.Fn lh_<type>_insert
returns the value being replaced.
.Dv NULL
is returned on normal operation and on error.
.Pp
.Fn lh_<type>_delete
returns the entry being deleted.
.Dv NULL
is returned if there is no such value in the hash table.
.Pp
.Fn lh_<type>_retrieve
returns the hash table entry if it has been found, or
.Dv NULL
otherwise.
.Pp
.Fn lh_<type>_error
returns 1 if an error occurred in the last operation, or 0 otherwise.
.Pp
.Fn lh_<type>_free ,
.Fn lh_<type>_doall ,
and
.Fn lh_<type>_doall_arg
return no values.
.Sh NOTES
The various LHASH macros and callback types exist to make it possible to
write type-checked code without resorting to function-prototype casting
\(em an evil that makes application code much harder to audit/verify and
also opens the window of opportunity for stack corruption and other
hard-to-find bugs.
It also, apparently, violates ANSI-C.
.Pp
The LHASH code regards table entries as constant data.
As such, it internally represents
.Fn lh_<type>_insert Ap ed
items with a
.Vt const void *
pointer type.
This is why callbacks such as those used by
.Fn lh_<type>_doall
and
.Fn lh_<type>_doall_arg
declare their prototypes with "const", even for the parameters that pass
back the table items' data pointers \(em for consistency, user-provided
data is "const" at all times as far as the LHASH code is concerned.
However, as callers are themselves providing these pointers, they can
choose whether they too should be treating all such parameters as
constant.
.Pp
As an example, a hash table may be maintained by code that, for
reasons of encapsulation, has only "const" access to the data being
indexed in the hash table (i.e. it is returned as "const" from
elsewhere in their code) \(em in this case the LHASH prototypes are
appropriate as-is.
Conversely, if the caller is responsible for the life-time of the data
in question, then they may well wish to make modifications to table item
passed back in the
.Fn lh_<type>_doall
or
.Fn lh_<type>_doall_arg
callbacks (see the "STUFF_cleanup" example above).
If so, the caller can either cast the "const" away (if they're providing
the raw callbacks themselves) or use the macros to declare/implement the
wrapper functions without "const" types.
.Pp
Callers that only have "const" access to data they are indexing in a
table, yet declare callbacks without constant types (or cast the "const"
away themselves), are therefore creating their own risks/bugs without
being encouraged to do so by the API.
On a related note, those auditing code should pay special attention
to any instances of DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros
that provide types without any "const" qualifiers.
.Sh INTERNALS
The following description is based on the SSLeay documentation:
.Pp
The lhash library implements a hash table described in the
.Em Communications of the ACM
in 1991.
What makes this hash table different is that as the table fills,
the hash table is increased (or decreased) in size via
.Xr OPENSSL_realloc 3 .
When a 'resize' is done, instead of all hashes being redistributed over
twice as many 'buckets', one bucket is split.
So when an 'expand' is done, there is only a minimal cost to
redistribute some values.
Subsequent inserts will cause more single 'bucket' redistributions but
there will never be a sudden large cost due to redistributing all the
\&'buckets'.
.Pp
The state for a particular hash table is kept in the
.Vt LHASH
structure.
The decision to increase or decrease the hash table size is made
depending on the 'load' of the hash table.
The load is the number of items in the hash table divided by the size of
the hash table.
The default values are as follows.
If (hash->up_load < load) => expand.
if (hash->down_load > load) => contract.
The
.Fa up_load
has a default value of 1 and
.Fa down_load
has a default value of 2.
These numbers can be modified by the application by just playing
with the
.Fa up_load
and
.Fa down_load
variables.
The 'load' is kept in a form which is multiplied by 256.
So hash->up_load=8*256 will cause a load of 8 to be set.
.Pp
If you are interested in performance the field to watch is
.Fa num_comp_calls .
The hash library keeps track of the 'hash' value for each item so when a
lookup is done, the 'hashes' are compared, if there is a match, then a
full compare is done, and hash->num_comp_calls is incremented.
If num_comp_calls is not equal to num_delete plus num_retrieve it means
that your hash function is generating hashes that are the same for
different values.
It is probably worth changing your hash function if this is the case
because even if your hash table has 10 items in a 'bucket', it can be
searched with 10
.Vt unsigned long
compares and 10 linked list traverses.
This will be much less expensive that 10 calls to your compare function.
.Pp
.Fn lh_strhash
is a demo string hashing function:
.Pp
.Dl unsigned long lh_strhash(const char *c);
.Pp
Since the LHASH routines would normally be passed structures, this
routine would not normally be passed to
.Fn lh_<type>_new ,
rather it would be used in the function passed to
.Fn lh_<type>_new .
.Sh SEE ALSO
.Xr lh_stats 3
.Sh HISTORY
The lhash library is available in all versions of SSLeay and OpenSSL.
.Fn lh_<type>_error
was added in SSLeay 0.9.1b.
.Pp
In OpenSSL 0.9.7, all lhash functions that were passed function pointers
were changed for better type safety, and the function types
.Vt LHASH_COMP_FN_TYPE ,
.Vt LHASH_HASH_FN_TYPE ,
.Vt LHASH_DOALL_FN_TYPE ,
and
.Vt LHASH_DOALL_ARG_FN_TYPE
became available.
.Pp
In OpenSSL 1.0.0, the lhash interface was revamped for even better type
checking.
.Sh BUGS
.Fn lh_<type>_insert
returns
.Dv NULL
both for success and error.
